Neurodevelopmental disorders affect 5% of children and are highly disruptive and impairing; Attention Deficit Hyperactivity Disorder (ADHD) and Autism Spectrum Disorder (ASD) together are the most prevalent and impairing of these disorders in childhood. Previously, affected individuals were thought to "grow out" of such disorders by adult life. However, we now know that is not correct. Longitudinal patient studies show that many manifest high symptom levels and impairment in adult life. However individual developmental trajectories are highly variable and symptom profiles change with maturation. Intriguingly, recent evidence suggests that some forms of adult ADHD emerge without being preceded by a similar childhood phenotype. These findings highlight the need to investigate the natural history of neurodevelopmental disorders into adulthood, in unselected population cohorts. Given the paucity of research on neurodevelopmental disorders and traits at a population level in adulthood, key questions are: 1) what is the longer-term natural history of neurodevelopmental disorders? 2) Do neurodevelopmental disorders originate early in development even when manifest in adult life?